124 CIRCULATING FLUIDS : 



act of metamorphosis represent exactly what we understand by 

 the vitality of the blood, 



Respiration in man and the mammalia is effected by the 

 dilatation and contraction of the cavity of the thorax. 



Since the diaphragm in a state of relaxation is arched, and 

 in a state of contraction during inspiration becomes flattened, 

 the cavity of the thorax is increased during inspiration, the 

 surface of the lung follows the retreating walls, its volume 

 becomes enlarged, and the atmospheric air rushes into its 

 cells. The branches of the air-tubes ramify to an extraor- 

 dinary degree in the parenchyma, and their most minute ex- 

 tremities terminate in vesicular dilatations, which do not com- 

 municate with each other, and whose walls are covered with 

 the peripheral capillary network. From a calculation of Lie- 

 berkuhn,' it would appear that the whole surface of the ramify- 

 ing air-tubes in man amounts to 1400 square feet, on which 

 extraordinary surface the blood and atmospheric air are in 

 contact with each other, (being separated merely by a moist, 

 permeable membrane,) and the former absorbs the required 

 amount of oxygen. 



Davy calculates that the human lung after the strongest 

 expiration, still contains 35 cubic inches of air; after an 

 ordinary expiration 108 cubic inches ; after an ordinary inspi- 

 ration 118, and after a very deep inspiration 340 cubic inches. 



In ordinary inspiration and expiration (about 26 or 37 

 in the minute) the amount of air that is changed varies from 

 10 to 13 cubic inches. 



According to Herbst, full-sized adidts usually inspire from 

 20 to 25 cubic inches; persons of smaller stature from 15 to 

 20. The volume of air inspired during each respii-atory act is 

 fixed by Allen and Pepys at 16-5, by Abilgaard at from 3 to Q, 

 and by Thomson at 40 cubic inches. 



The quantity of air that enters the lungs in the course of 

 24 hours is calculated by Davy at from 400,000 to 500,000 

 cubic inches, by Allen and Pepys at from 460,800 to 475,200, 

 and by Thomson at as much as 1,152,000, or 52*5 pounds, the 

 respirations in this case being 20 in the minute. ^ 



Atmospheric air once respired is lessened in volume; and 



' Scliultz, op. cit. ]). 288. 



^ Ginelin's Handhucli dcr theoretischen Chemie, vol. 2, p. 1519. 



